1. Field of the Invention
The present invention relates to a method of a rapid initial synchronization for pseudonoise (PN) codes in the receiver of a code division multiple access (CDMA) system. A plurality of PN codes and multiple symbol differential detection scheme are used to achieve an initial synchronization in mobile communication systems.
2. Description of the Prior Art
CDMA systems include a transmitter adapted to transmit a signal after spreading the band of the signal using a PN code and a receiver adapted to search a code pattern identical to the PN code of a signal received therein and to despread the band of the received signal, thereby recovering an original signal. In such CDMA systems, it is important to provide a timing between the PN produced in the receiver and the PN received in the receiver. This timing procedure is called a "synchronization procedure". Such a synchronization procedure is classified into an initial synchronization procedure, in which a coarse alignment at an interval of one chip is achieved, and a synchronization tracking procedure, in which a fine alignment at an interval of less than one chip is achieved. The present invention is associated with the initial synchronization procedure. Where an initial synchronization method according to the present invention is utilized, it is possible to achieve a rapid initial synchronization, as compared to conventional serial search methods.
Generally, an initial synchronization may be obtained, using one of three systems, namely, a serial search system, a sequential estimation system and a matched filter system. Commercial CDMA mobile communication systems mainly adopt the serial search system because it provides a simple hardware configuration and a superior performance even at a low signal-to-noise ratio, even though it requires a lot of time for the initial synchronization procedure. This system has a configuration shown in FIG. 1.
In the serial search system shown in FIG. 1, a received signal in a receiver is multiplied by a code pattern which is a reference signal generated from local PN code generator, for a window size corresponding to a search period. The above operation is done for the entire period of 2.sup.n -1 while shifting the reference signal by one clock and accumulating the result of every operation. Thereafter, a code pattern having a largest correlation value Vs is searched for from the accumulative values. Using this code pattern, a synchronization between the received signal and the reference signal is achieved. FIG. 2 illustrates a received PN code and a generated PN code in the receiver, where "k" and "k'" represent phase offsets of the received and locally generated PN codes, respectively. "M" represents a correlation length (window) between the two signals. When the phase offset k of the received PN code is identical to the phase offset k' of the locally generated PN code, the correlation value Vs between the two PN codes is maximized, i.e., an initial synchronization is acquired.
The correlation value Vs between the two PN codes is expressed as follows: EQU Vs=(S.sub.k +n.sub.k).times.S.sub.k' +(S.sub.k+1 +n.sub.k+1).times.S.sub.k'+1 + . . . +(S.sub.k+M +n.sub.k+M).times.S.sub.k'+M (1)
where, "S.sub.k, . . . , S.sub.k+M ", "S.sub.k', . . . , S.sub.k'+M ", "n.sub.k, . . . , n.sub.k+M " and "M" represent received PN codes, locally generated PN codes, Additive White Gaussian Noise and a window size respectively.
In the above-mentioned system, the acquisition time Tacq taken for an initial synchronization is expressed as follows: EQU Tacq=PN Code Length.times.Window Size/PN Code Rate (Second)(2)
Where this equation is applied to IS-95 (Interim Standard-95) system commercially available as a CDMA digital mobile communication system, an acquisition time Tacq of about 1.7 to 3.4 seconds is taken for the initial synchronization because the system uses a PN code rate of 1.2288 Mcps (chip per second), a PN code interval of 2.sup.15 -1 (32,767), and a window size of about 64 to 128.
Meanwhile, for future mobile communication systems, wideband CDMA techniques use a relatively long PN code. For instance, the IS-95 system uses a short PN code of 2.sup.15 -1 whereas the wireless local loop (WLL) standard system adopted in Korea uses a long PN code of 2.sup.32 -1. The WLL system of Interdigital Corporation (IDC) uses a longer PN code of 2.sup.36 -1. Such a long PN code, however, is partially used in each base station. That is, each base station uses a portion of the long PN code corresponding to a certain interval of time. For example, the WLL system uses only a 20 ms portion of the long PN code of 2.sup.32 -1. In the case of a system using a PN code rate of 8.192 Mcps (10 MHz), the code length used in each base station corresponds to 163,840 chips (8.192 Mcps.times.20 ms=163,840 chips). However, it is hard to know in advance information about which 20 ms portion of the long PN code of 2.sup.32 -1 is used in any base station. As a result, in the case of serial search, there is a limitation in that all portions of the PN code should be searched for in order to obtain an initial synchronization of the PN code. A lengthened acquisition time for the initial synchronization, moreover, is required in the conventional serial search methods because a high PN code rate of about 10 Mcps is used. For example, even at a window size of 64, the following lengthened acquisition time for the initial synchronization is taken: ##EQU1##
Consequently, it is hard to apply the serial search methods to practical systems in which a PN code acquisition should be achieved within a few seconds.